Appraisal Advice! How to evaluate an irregular new build

[Jimmy O'Connor]

I've been working on an acquisition of a vacant triangular shaped lot on the southern border of center city in Philly. I have an investor that does new builds in already established neighborhoods to rent them out, regardless if it is zoned RM1 or RSA5. The hold up is how the property would refinance to pull all of the money out. 

Once built, the structure should have roughly 1900 interior square feet with an 18 foot frontage that comes to a narrow point towards the back of the lot. Based on square footage, it should comp out to roughly 530-560k BUT that is comparing to normal 15-16 footers that are normal rectangular shaped. I'm having trouble quantifying how much of a price cut the shape would make to a refinance since the difference in value seems very subjective.

APPRAISERS PLEASE HELP!

How would you compare a triangular/trapezoidal shaped property on a triangular lot to a typical new build? Are square footage, proximity of comps, and finishes still take precedence?